Microwave filters generally are designed to be efficient and compact. Efficiency can be characterized either as low loss or high quality factor. Compact size of the filter is necessary for those applications in which a number of filters are proximately located in a limited space, viz., frequency multiplexers or demultiplexers. Additionally, the filters are designed to minimize interference, such as interresonator coupling, among proximately located filters.
Microwave filters known in the art have been designed using either cavity resonators (see N. Ehrlich et al., "Cell-Site Hardware," The Bell System Technical Journal, Vol. 58, Jan., 1979, pp. 153-199), dielectric resonators or the like. Higher quality factors result from the use of dielectric resonators in the microwave filters. Ceramic dielectric resonators made from barium titanate, Ba.sub.2 Ti.sub.9 O.sub.20, as shown in U.S. Pat. No. 3,938,064 issued to H. M. O'Bryan, Jr. et al. on Feb. 10, 1976, exhibit higher quality factors than corresponding cavity resonators. Therefore, it appears that the dielectric resonator is a more efficient microwave filter.
Dielectric resonators are excited by electromagnetic radiation at a resonance frequency of the dielectric resonator. Emissions from excited dielectric resonators interfere with and possibly excite other proximately located dielectric resonators. This type of interference phenomenon is called interresonator coupling. Housings, separately enclosing each dielectric resonator and designed to accommodate a particular mode and frequency of electromagnetic propagation, substantially eliminate interresonator coupling. However, these housings can decrease the efficiency of the microwave filter because of electromagnetic coupling between the housing and the excited dielectric resonator.
Prior theoretical electrical optimization of a housing which is electromagnetically coupled to a dielectric resonator normally results in a housing having a prescribed shape. Although this prior housing possesses optimum electrical characteristics, the housing is prohibitively large and impractical for use in applications involving several filters in a limited space. Hence, electrical optimization is in conflict with size reduction of the microwave filter using the dielectric resonator.
In one example, a housing shaped as a right circular cylinder encloses a similarly shaped dielectric resonator, concentrically located within the housing, for supporting a transverse electric propagation mode such as TE.sub.01.delta.. Electrical optimization of a microwave filter incorporating the exemplary housing yields a housing whose diameter is at least twice as large as the diameter of the dielectric resonator. The resulting size of the housing severely restricts the number of microwave filters which can be located in a limited space. Therefore, this electrically optimized microwave filter is impractical for use in applications, where size of the microwave filter is an important criterion, such as frequency multiplexers or demultiplexers.